Trump said there had been "tremendous progress" after his "firm" warnings during the tense two-day meeting in Brussels, during which he singled out Germany for special criticism.

"I let them know I was extremely unhappy with what was happening and they have substantially upped their commitment and now we're very happy and have a very, very powerful, very very strong NATO, much stronger than it was two days ago," Trump said in a freewheeling press conference.

"Tremendous progress has been made, everyone's agreed to substantially up their commitment they're going to up it at levels they've never thought of before. It's been amazing to see the level of spirit in that room."

French President Emmanuel Macron, who has cosied up to Trump despite their wide political differences, said NATO was "much stronger" after the summit.

Trump threw the summit into turmoil by demanding not only that allies reach their commitment to increase spending to two percent of GDP "immediately" -- instead of by 2024 as previously agreed -- but also telling them to eventually double the figure to a punishing four percent.

NATO Secretary General Jens Stoltenberg called an emergency session of all 29 allies to address Trump's demands, and the US leader said his hectoring had paid off.

"The commitment was at two percent and that'll be going up quite a bit higher than that," he said.

- Putin 'not an enemy' -

After an opening day of summit talks marked by clashes between Trump and German Chancellor Angela Merkel, NATO leaders had hoped to focus on policy on Ukraine and Afghanistan.

But just days out from a high-stakes meeting with Russian President Vladimir Putin in Helsinki, the mercurial US leader threatened to throw the transatlantic alliance into disarray.

With allies fearing he could strike some sort of deal with NATO's key adversary, Trump said that he saw Putin as a "competitor" but not an "enemy".

Trump insisted that he would bring up key subjects including Syria, although he once again failed to rule out recognising Russia's annexation of Crimea.

He also vowed to raise allegations of Russian meddling in the 2016 US presidential election, amid an ongoing US investigation into whether his campaign colluded with Moscow.

Trump himself accused Germany of being a "captive" of Russia due to a multi-billion-dollar pipeline deal, complaining that Germany and other NATO allies "pay only a fraction" of the cost of defending Europe.

Trump had explicitly linked NATO with a transatlantic trade row by saying the EU shut out US business while expecting America to defend it, singling out Germany for particular criticism.

"Germany just started paying Russia, the country they want protection from, Billions of Dollars for their Energy needs coming out of a new pipeline from Russia. Not acceptable! All NATO Nations must meet their 2% commitment, and that must ultimately go to 4%!" Trump tweeted on Thursday morning before the second day of talks.

Apart from the US, only three NATO countries hit the two-percent target in 2017 -- Britain, Greece and Estonia -- but four more are expected to clear the threshold this year.

- Trump Brexit doubts -

Trump has said next week's meeting with Putin "may be the easiest" part of his European tour.

Trump heads to Britain on Thursday, where the government is in crisis over Brexit and where tensions with Russia have spiked after London blamed Moscow for the death this month of a British woman from contact with the Novichok nerve agent.

Trump poured fuel on the Brexit fire just hours before he was due to land there, saying he was not sure that Britons had voted for the Brexit plan presented by Prime Minister Theresa May, which has triggered a British cabinet rebellion.

"The people voted to break it up (Britain's ties with the EU)," Trump told a press conference in Brussels on Thursday.

"So I would imagine that's what they will do, but maybe they will take a little bit of a different route. I don't know that is what they voted for."